This invention relates to digital radio communication systems employing multiple sub-carriers, and more particularly to dynamic use of sub-carriers within such systems.
In a digital radio communication system a base station transmits a signal at a transmission rate to a remote unit through a radio channel having channel characteristics, such as an attenuation. The signal is transmitted using transmission parameters, such as a modulation level and a coding rate. The transmission rate depends on the transmission parameters. The transmission parameters are constrained by an acceptable bit error rate and by a signal to interference ratio of the signal, the latter varying in time with the channel characteristics. The communication system can use adaptive modulation to adjust the transmission parameters to accommodate changes in channel characteristics over time. If a change in channel characteristics results in a lower signal to interference ratio, the modulation level must be reduced (for example, from 16-QAM to QPSK) or the coding rate must be improved (for example, from xc2xe to ⅔) in order to maintain the acceptable bit error rate, albeit at a lower transmission rate. If a change in channel characteristics results in a higher signal to interference ratio, the base station can increase the modulation level or decrease the coding rate in order to obtain a higher transmission rate.
In a communication system that implements adaptive modulation, the base station and the remote unit must be synchronized with respect to the transmission parameters. In current communication systems the remote unit determines a channel quality when the remote unit receives a frame of data. The remote unit may estimate, for example, the signal to interference ratio of the channel. The remote unit sends a signal back to the base station reporting the channel quality. Using the channel quality report received from the remote unit, the base station calculates a set of optimum transmission parameters which the base station will use in its next transmission of data. However, the base station must first send the set of new optimum transmission parameters to the remote unit using the previous transmission parameters. The remote unit receives the set of new optimum transmission parameters, interpreting the signal using the previous transmission parameters. The remote unit then decodes subsequent frames of data using the new optimum transmission parameters.
In communication systems that make use of multiple antennae for transmission and reception, the transmission parameters may include adaptive antenna and coding parameters. For example, some xe2x80x9csmart antennaxe2x80x9d systems may adaptively adjust their directional patterns towards the remote units. An outline of such systems may be found in the paper by J. H. Winters, xe2x80x9cSmart Antennas for Wireless Systemsxe2x80x9d, IEEE Pers. Commun., vol. 5, no. 1, February 1998, pp. 23-27, which is incorporated by reference herein. Similarly, the radio system may make use of the multiple communication channels that exist between transmitters and receivers with multiple antennae. In this case, the transmission parameters include both space (across multiple antennae) and time (different time of transmission) aspects that adapt the transmissions to the multiple propagation environment. An outline of such systems may be found in the paper by A. J. Paulraj and B. C. Ng, xe2x80x9cSpace-time Modems for Wireless Personal Communicationsxe2x80x9d, IEEE Pers. Commun., vol. 5, no. 1, February 1998, pp. 36-48, which is incorporated by reference herein.
In communication systems employing many sub-carriers, such as those that employ Orthogonal Frequency Division Multiplexing (OFDM systems), the channel quality may vary with the frequency of each sub-carrier. OFDM systems can use 1000 sub-carriers, and transmission of information describing the channel quality and a set of optimum transmission parameters for each sub-carrier would require significant overhead, reducing the efficiency of the communication system. In current methods, the signal to interference ratio is averaged over all sub-carriers so that only one signal to interference ratio is reported to the base station and only one set of new optimum transmission parameters is transmitted to the remote unit. In this method, the single set of new optimum transmission parameters results in an unnecessarily low transmission rate for individual sub-carriers whose signal to interference ratio is higher than the average signal to interference ratio reported by the receiver.
The present invention provides a method of selecting and signalling the identity of acceptable groups of sub-carriers in a radio communication system. A remote unit receives a signal as more than one sub-carrier signal from a base station. The remote unit determines a channel quality (such as a signal to interference ratio or a reciprocal of an error rate) of each group of sub-carrier signals, and compares the channel quality of each group of sub-carrier signals with a threshold. A sequence of numbers is generated, there being one number for each group of sub-carrier signals. Each number has a value belonging to a first set of values if the channel quality of the corresponding group of sub-carriers is above the threshold, and has a value belonging to a second set of values if the channel quality of the corresponding group of sub-carriers is not above the threshold, the two sets of values having no values in common. The first set of values may consist of the value one and the second set of values may consist of the value zero, in which case each number in the sequence has a length of one bit. The remote unit generates at least one value by which the base station can determine one or more Link Modes, a Link Mode being a set of transmission parameters. The remote unit transmits the sequence of numbers and the values by which the base station can determine the Link Mode or Link Modes.
The remote unit may calculate the average channel quality of groups of sub-carriers whose channel quality is above the threshold, in which case the average channel quality is transmitted to the base station. The remote unit may also determine a Link Mode using the average channel quality, in which case the Link Mode is transmitted to the base station. The remote unit may alternatively determine a Link Mode for each group of sub-carriers whose channel quality is above the threshold, in which case the sequence of numbers and the values by which the base station can determine the Link Mode of each sub-carrier whose channel quality is above the threshold is transmitted. In the latter case, the sequence of numbers and the values by which the base station can determine the Link Modes can be combined into a single sequence of numbers.
The present invention also provides a method of assigning transmission tasks to at least one sub-carrier in a radio communication system. A base station receives a return signal, and extracts from the return signal a sequence of numbers, each number corresponding to one group of sub-carriers, and at least one value by which the base station can determine at least one Link Mode. The base station determines at least one Link Mode based on the at least one value. The base station defines a set of acceptable groups of sub-carriers as all groups of sub-carriers for which the corresponding number has a value belonging to a first set of values, and defines a set of unacceptable groups of sub-carriers as all groups of sub-carriers for which the corresponding number has a value belonging to a second set of values, the two sets of values having no values in common. The base station allocates for data transmission at one of the Link Modes the sub-carriers which belong to the groups of sub-carriers within the set of acceptable groups of sub-carriers. In one embodiment, the return signal includes an average channel quality and the base station determines a single Link Mode based on the average channel quality. In another embodiment, the return signal includes a reference to a Link Mode and the base station determines a single Link Mode based on the reference to the Link Mode. In yet another embodiment, the return signal includes references to one Link Mode for each acceptable group of sub-carriers, possibly within the sequence of numbers, and the base station determines a Link Mode for each acceptable group of sub-carriers based on the corresponding reference. The base station may allocate for low sensitivity data transmission sub-carriers within some of the unacceptable sub-carriers, may allocate for data transmission at a low transmission rate sub-carriers within some of the remaining unacceptable sub-carriers, and may divert transmission power from the remaining unused unacceptable sub-carriers to other sub-carriers.
The method provides improved efficiency of a communication system by allowing sub-carriers having a high signal to interference ratio to use a higher transmission rate. Sub-carriers having a low signal to interference ratio can be used for less sensitive traffic, or their transmission power can be diverted to sub-carriers having a high signal to interference ratio.